This application relates to infrared spectroscopy and, more particulary, to optics included in an infrared spectrophotometer for directing source infrared radiation to a sample under analysis and thence directing resultant radiation as modified by the infrared absorption characteristics of the sample to a detector. Specifically, the invention is directed to an optical system including a cylindrical internal reflection element for incorporation into an infrared spectrophotometer for facilitating the analysis of liquid and fluidized samples, especially aqueous solutions, by infrared spectroscopy.
In the infrared range, practically all organic (and many inorganic) molecules have characteristic spectra that can positively identify them. Unlike chromatographic techniques (and, for that matter, most other analytical methods), infrared (IR) analysis is nondestructive and, with the aid of a computer, produces qualitative and/or quantitative information almost instantly. In addition, IR analysis is a universal technique since it yields spectra on almost any form of a material (solid, liquid, or gas) directly, without changing the material's physical state
Much quantitative infrared analysis is conducted by means of Fourier Transform IR analyzers (FTIR's). Computer assisted spectroscopy, including spectroscopy by means of FTIR's, is most important to the analysis of aqueous solutions due to the ability to accurately subtract out the water background.
The known art includes the following for performing analysis of strongly absorbing liquids by means of infrared spectroscopy: very short path length transmission cells; flat-plate multiple internal reflection cells; probes; cylindrical internal reflection elements using Cassegrain optics; rod crystals using refractive lenses; and rod crystals using funnel-shaped mirrors. There are problems with each of these.
IR analyzers have been used for many years for on-stream gas analysis. But liquids, unlike gases, have closely packed molecules, and they absorb infrared radiation strongly. While infrared gas transmission cells commonly are 10 centimeters to several meters in length, liquid transmission cells are typically less than a millimeter, sometimes as short as 10 micrometers, in thickness in order to allow transmission of sufficient infrared radiation to be measurable. Tightly packed molecules in a liquid with water absorption bands make it necessary to go to extremely short path lengths in order to achieve transmission. Flow rates are very limited for flow-through liquid transmission cells. Moreover, flow-through transmission cells have very poor flow characteristics and high back pressures. It is not practical to attempt to pass a process-stream loop continuously through such flow-through transmission cells. Very short path length transmission cells are difficult to fill, especially in the case of a high viscosity liquid. They are difficult to clean, and cleaning takes a significant amount of time. Slight changes in pressure and/or temperature cause changes in path length. Path lengths change with viscosity and over time. Transmission cells often leak. Usually non-inert amalgam metal transmission cells are used.
In recent years, a technique called multiple internal reflection (MIR) has come into use, which overcomes the need for very short path length transmission cells. Originally developed for obtaining infrared spectra on solid surfaces, the MIR technique also can be used for strongly absorbing liquids, because the effective path lengths generated in MIR cells are typically in the 1- to 50-micrometer range.
The MIR technique makes use of the fact that when infrared radiation propagating within a transparent medium is reflected internally from a surface, a portion of the radiation projects slightly beyond the reflecting surface. When a rare medium, such as air, is in contact with the surface, there is little interference with or attenuation of the infrared radiation. But, if an absorbing liquid is brought into contact with the surface, infrared radiation is absorbed at those wavelengths at which the liquid normally absorbs (as in a transmission cell). Therefore, the reflected infrared radiation carries information that indicates absorption by the sample in the region of the reflecting surface. Since the infrared radiation extends only a few micrometers into the sample, any sample beyond that distance has no effect on the measurement.
Typically, the optical element through which the infrared radiation propagates is a flat plate with polished parallel surfaces and angled ends. A sample is placed in contact with one or both of the reflecting surfaces. Absorption at specific wavelengths then is measured to provide composition information for the sample.
The flat-plate configuration has limitations, however, in that the rectangular shape of the flat-plate element makes it difficult to seal the element into a leak-tight chamber without causing substantial optical interference from the seal. IR analysis using the flat-plate configuration is also characterized by the following additional difficulties. FTIR beams are round, and, consequently, slight movements cause substantial differences in infrared radiation throughput. Good seals are difficult to obtain, especially if pressure is present. Slightly different alignment can cause sampling of a different area. Either the entire MIR cell or the flat-plate element must be removed to clean, thereby resulting in a need to re-align.
IR analysis using probes is difficult because tolerances on probes are extremely critical. Also, good seals are difficult to effect on probes, because they generally have a square cross-section.
An MIR optical element constructed from a cylinder instead of a flat plate is disclosed in Paul A. Wilks, Jr., "Sampling Method Makes On-Stream IR Analysis Work," Industrial Research & Development, September, 1982. Polished cone-shaped ends on the cylindrical element perform the same function as the angled ends on a flat-plate element.
The cylindrical configuration has significant advantages over known MIR cells having flat plates. The cylindrical element can be sealed into a sample chamber capable of withstanding several atmospheres of pressure, yet a sample can flow freely through the chamber. Furthermore, the seal has only a small area of contact with the cylindrical element; therefore, the seal has little effect on the infrared radiation within the element. Additionally, the only materials exposed to the sample are the glass (or stainless steel) envelope which surrounds the cylindrical element, the O-rings generally used for sealing, and the element itself, which typically is constructed from a material having a high index of refraction, such as zinc sulfide or sapphire. The MIR cell can be flushed with caustic solution or with organic solvents for cleaning. For aqueous solutions, flushing with water is sufficient for cleaning.
The Wilks article discloses large-aperture optics included in an IR analyzer, which optically couple with the cone-ended cylinder. The optics include a two-mirror configuration. Infrared radiation passes through a hole in a first mirror and strikes a second mirror with central rays at an angle of 45.degree.. The outside polished surface of the first mirror defines the maximum angle, and the diameter of the hole defines the minimum angle, of the edge rays. The radius of curvature of the second mirror is selected so that the infrared radiation is reflected to the first mirror as a beam that is approximately collimated. Since the first and second mirrors are identical, the first mirror forms an image of the source a short distance through a hole in the second mirror.
When the 45.degree. cone-shaped end of the cylindrical element nearer the source end of the IR analyzer is placed so that the image of the source is just inside the cone, the infrared radiation which enters the element is at an average angle perpendicular to the cone face. Therefore, the infrared radiation reflects down the cylindrical element at an average internal angle of 45.degree..
When the infrared radiation reaches the cone-shaped end of the cylindrical element nearer the detector end of the IR analyzer, the radiation emerges as an expanding ring of radiation with the same angular spread as the radiation which enters the element. Consequently, a pair of mirrors identical to those at the source end of the IR analyzer can be used for focusing the infrared radiation onto the detector.
The optics disclosed in the Wilks article, however, are configured for a stand-alone IR analyzer in which the infrared radiation from the source is highly divergent. Consequently, the optics, although they have proven useful in the case of divergent sources, are not effective in the case of a collimated or convergent beam, such as in the case of an existing center focus or edge focus IR analyzer.
Other optical systems have been used in combination with an MIR cell having a cylindrical element for use with collimated or convergent beams. However, the optics have proven inadequate for various reasons.
One known MIR cell having a cylindrical element includes Cassegrain optics. Unfortunately, Cassegrain optics have the difficulty that the physical size of the optics is very large, and, consequently, the MIR cell is unuseable in many IR analyzers. Cassegrain optics are very difficult to align and use due to size and magnification. In addition, the use of Cassegrain optics results in significantly lower infrared radiation throughput due to obscuration loss from a necessarily large secondary mirror. (Conventional Cassegrain optics design assumes a continuous secondary surface.)
Another known MIR cell having a cylindrical element includes optics in the form of refractive lenses as disclosed in Wilks, Jr., U.S. Pat. No. 3,370,502. However, lenses have unacceptable inherent losses, as well as a limited frequency range. Furthermore, lenses have a wide variation in angle of incidence, which creates significant sampling problems.
Also, a known MIR cell includes optics comprising funnel-shaped mirrors (single mirror elements) in combination with a cylindrical element as disclosed in Wilks, Jr., U.S. Pat. No. 3,370,502. This configuration is indicated to be particularly suited for incoming parallel rays (collimated source infrared radiation), such as in spectrophotometers. However, the funnel-shaped mirror optics undesirably have a wide variation in angle of incidence. Furthermore, difficulties arise in focusing the emergent infrared radiation onto the detector.
The optics used in the past in combination with a cylindrical element for concentrating and/or collecting infrared radiation as part of an MIR cell in infrared spectroscopy with collimated or converging source radiation, such as present in the sample chamber of an FTIR spectrophotometer, have not met with any degree of success. The present invention provides optical means which in combination with a cylindrically shaped internal reflection element forms an optical system for an MIR cell which not only overcomes the problems encountered with similar cells in the past, but produces a throughput of two to six times greater than known MIR cells having a similar internal reflection element, when the infrared radiation source is a collimated or convergent beam with a circular or near circular cross-section.